Single cell protein (Single cell protein, SCP), also known as microbial protein or bacterial protein, is made by the single cell organisms such as cultured yeast, non-pathogenic bacteria, micro-bacteria, fungi after purification and drying using the industrial waste water, waste gas, natural gas, petroleum hydrocarbons, agro-processed products and organic wastes as the medium. It is an important source of protein in food and feed industries. SCP products are rich in protein, with full range of amino acids at appropriate proportions. SCP is also rich in fats, carbohydrates, nucleic acids, vitamins and inorganic salts, and contains a variety of mineral elements such as calcium, phosphorus, potassium, iron, magnesium, sodium, manganese, and a variety of enzymes. Yeast, yeast hydrolyzates have been successfully used as substitutes of fish meal protein in the animal feeds, which can enhance animal immunity and improve the taste of feeds, etc., SCP nucleotide and its derivatives have very important physiological and biochemical functions, mainly as follows: the raw material of RNA and DNA synthesis; the regulatory substance of metabolism; the ingredients of coenzyme, such as coenzyme A; important intermediary for activation of glycogen and glycoproteins synthesis, the intermediary of phospholipid synthesis, activation of intermediate products; participation in genetic information transfer; ATP is the direct energy of life process; AMP can regulate the smooth muscle contractility and blood flow; and nucleotides can be used as the donor of methyl, etc.
Since the harmful microbes, harmful chemicals, heavy metals may influence the product quality and safety, there are fewer varieties of SCP products that have been developed or are under industrial development. It mainly focuses on the culture and production of SCP using the product or by-product from the food industry with low safety risk as the raw material, for example, the use of beer yeast and zymolytic beer yeast as feed ingredients in animals. The beer production process should meet the strict food safety standards, and the biological, chemical and physical hazards of organic byproducts and yeast proteins produced should be minimized, to provide reliable safety guarantee for its applications in the feed industry.
Patent WO2009059163A1 provides a method for the culture of SCP. The cultured SCP can be used as excellent protein source of animal feeds. This technology adopts the wastewater producing in the food or beverage processing factory such as brewery as the liquid culture medium of the bacteria. The beverage wastewater has rich organic matter, nitrogen and phosphorus and other nutrients, and has no contamination of harmful microorganisms, toxic chemicals. Although these wastewater is of no value for the beverage factory and the treatment of the wastewater containing organic matter and nitrogen and phosphorus will cost much, the wastewater nutrients can become nutrient-rich medium from the perspective of the microbial culture, which can be used to produce special SCP. According to the patent WO2009059163A1, the SCP products with high protein content and high nutritional value are produced with the genetically modified bacteria through adjusting the ratio of various nutrients in the wastewater. The SCP is a mixture of bacteria, mainly containing Micrococcus, Bacillus bacteria, nitrifying bacteria, Alcaligenes and other aerobic bacteria. Meanwhile, the nutrient content in the beverage wastewater will be reduced after culture of microbes, which reduce the level of water pollution. This patented technology can achieve the production of high-quality protein and reduce the concentration of organic matter in wastewater to below the legal standard, having both economical and environmental values. Thus, this technology has a very good industrial prospect. Since the bacterial growth cycle is short and the nutrient use efficiency is high, the yield of SCP culture is very high, which provides an economic guarantee for the industrialization. However, the solids content is very low in the culture due to the bacterial growth characteristics, usually lower than 4000 mg/L. If the SCP is processed into the finished product, it is necessary to consider how to extract the SCP from water and process them to dry product with low moisture content at a low cost. Based on the characteristics of SCP as described in the patent WO2009059163A1, the existing processing technology has the following drawbacks:    1) The processing technology only uses the SCP as ordinary protein for the production, mainly focuses on the protein dehydration, drying, sterilization, to produce the protein, without considering that the bacteria in the patent WO2009059163A1 can be used to produce high quality of product using further processing techniques;    2) The SCP cultured by the patent WO2009059163A1 contains a large amount of endogenous enzymes after study, specifically including nuclease, protease and lysozyme. These enzymes can decompose macromolecular nutrients. Therefore, by using the autolysis technique, a number of active substances can be decomposed from the endogenous enzymes of bacteria, such as free amino acids, nucleotides, monosaccharides and so on. However, according to the existing process, the direct drying and microwave sterilization after dewatering has destroyed most of the endogenous enzymes, therefore, it will not produce active substances;    3) The protein digestibility is still not high based on the existing process, which need to be further improved;    4) SCP has high content of nucleic acids. The nucleic acids are not decomposed to nucleotides and still in the cell after ordinary drying process. But through a certain method, nucleic acids can be decomposed to nucleotides; and SCP can have the food calling and immunity effect when used as the animal feeds. The existing process fails to consider how to decompose nucleic acids to nucleotides with high value.    5) Although the yield of cultured SCP is very high, the production capacity of the subsequent processing equipment still can be significantly higher than that of the cultured SCP, for example, in the patent CN2014100556582, restricted by the cultured low output and even cannot reaching the average capacity of equipment, which causes the wastes of the invested equipments.
Chinese Patent (No. 201410055658.2) discloses a method for producing SCP powder using biological mud. It is a recycling method with biological mud to produce SCP. After concentration of the biological mud discharged from biological farming system, then centrifugal dewatering, instant hot air drying, cooling and microwave sterilization are performed, to get crude SCP powder. This patent is a prior application of our company, but the cell protein utilization efficiency is low due to lack of autolysis process. Autolysis refers to a process that in vivo macromolecular substances of bacteria are hydrolyzed to amino acids, nucleotides, monosaccharide and other small molecules by using their own endogenous enzymes (proteases, nucleases, lysozyme, etc.). Autolysis is a complex process, affected by a variety of factors, including the bacterial species, moisture content, temperature, pH, time, type of endogenous enzymes, etc., More importantly, autolysis is not a single process under the natural condition. It is a process before dead living body becomes decayed. The macromolecules are decomposed to amino acids, nucleotides and other nutrients after autolysis, which provides important source of nutrition for the growth of spoilage microbes. Therefore, the results of autolysis are different due to the species of microbes, autolysis conditions, and target products, etc., moreover, the autolysis failure will result in the production of spoilage products such as ammonia, hydrogen sulfide, trimethylamine, etc.
Chinese patents (patent numbers 2008800240991 and 2011102810796) disclose the production of yeast autolysate using the autolysis process, to improve the product flavor. But the two patents are not applicable to this patent. Raw materials used in the two patents are anaerobic bacteria, which have greatly different characteristics from aerobic bacteria, and their autolysis ways are also different, for example, the time of autolysis of anaerobic bacteria is longer, and enzymes should be added to promote autolysis; autolysis of anaerobic bacteria is restricted by reaction vessel, moisture content, pH, temperature, etc., and the products are difficult to control, easy to produce high content of decayed products.